The porous bottom culture dishes (PBCDs) developed in this laboratory for the study of epithelial cells grown in confluent layers are now being used in more than 500 laboratories world- wide. This large utilization has resulted in part from the fact that PBCD are now commercially available from 2 sources (Millipore Corp. and Costar Corp.) with several types of porous membranes with various cell culture treatments and coatings. Many different types of epithelial and some endothelial and smooth muscle cells are being studied using the PBCDs. The objectives of the research projects being undertaken with the PBCDs go far beyond anything we had in mind at the outset of our development of them. The PBCDs made with collagen membranes also made in this laboratory still provide the best optical properties (for phase microscopy) of all the "homemade" and commercial PBCDs. Many cell types grow well on the collagen also. In fact, endothelial cells have been grown on one of the collagen membrane and smooth muscle cells on the other side. This makes a good model of a blood vessel. The devices developed in this laboratory to sterilly study sodium transport by confluent layers of epithelial cells grown on the PBCDs are being used in many laboratories. These are how being used for indications of confluency of layers of other cell types. The major role played by Ca in the regulation of many cellular processes has prompted us to improve methods for measuring free Ca activity in cell systems. We have developed a 2 mm diameter electrode using a hydrophobic porous membrane and neutral carrier Ca exchanger which has a response time of 10 seconds and a resistance of 10 megohms. It is useful for measuring small quantities of solutions representative of cell interiors for many biochemical studies. These electrodes have also been useful in measuring interesting Ca++ activity reductions in perfusion solutions which apparently result from ion pairing of calcium and bicarbonate ions.